1. Field of the Invention
The invention relates to laser diode modules having a laser diode, an optical fiber, and optical parts by which light emitted from the laser diode is optically coupled with the optical fiber; converging parts for transmitting light emitted from a laser diode into an optical fiber; and methods of optical coupling laser light and an optical fiber.
2. Description of the Prior Art
FIG. 1 is a cross-sectional view of a conventional laser diode module using a coupling method of a semiconductor laser and an optical fiber, for example, such as disclosed in Japanese Patent Publication No. Hei 4-66324. FIG. 1 shows a laser diode 1; a submount 2 on which the laser diode 1 is mounted; a photodiode 3; a photodiode carrier 4 for supporting the photodiode 3 so that the surface of the photodiode 3 and an optical axis of light emitted from the back side of the laser diode 1 intersect at an angle; a ceramic substrate 5; a base 6 for supporting the submount 2, the photodiode carrier 4, and the ceramic substrate 5; a collimating lens 7 for collimating light emitted from the front side of the laser diode 1; a collimating lens holder 8 for supporting the collimating lens 7; an enclosure 9; a signal pin 10 piercing the enclosure 9; a high-frequency electric line 11 formed on the ceramic substrate 5 for supplying power to the laser diode 1; gold wires 12-15 for connecting the power supply system from the signal pin 10 to the laser diode 1; pads 16 and 17 for bonding gold wires 13-15; a window 18 for preserving the airtightness of the enclosure 9 while allowing the transmitting of light emitted from the front side of the laser diode 1; an optical axis 19 of laser light emitted from the laser diode 1; a converging rod lens 20; a rotation axis 21 of the converging rod lens 20 which is cylindrical in shape the axis 21 being normal to the bases of the rod lens 20; a lens holder 22 joined to the enclosure 9 for holding the converging rod lens 20; an optical fiber 25; a core 23 of the optical fiber 25; a ferrule 24 for fixing and supporting the core 23 and the optical fiber 25; and a ferrule holder 26 for supporting the ferrule 24.
In a laser diode module using such a conventional coupling method of a semiconductor laser and an optical fiber, the converging rod lens 20 is placed so that the axis 21 is parallel to the optical axis 19 of light emitted from the front side of the laser diode 1 which is collimated by the collimating lens 7. As a result, the converging rod lens 20 converges light emitted from the front side of the laser diode 1 and changes the direction of the light's optical axis after it has traveled through the window 18. Converged light from the converging rod lens 20 travels into the core 23 of the optical fiber 25 at an angle depending on an angle of an end of the core 23 of the optical fiber 25 thereby achieving a high coupling efficiency. That is, laser light from the laser diode 1 can be coupled with the optical fiber with a high coupling efficiency.
In addition, the photodiode carrier 4 supports the photodiode 3 so that the surface of the photodiode 3 and the optical axis of light emitted from the back side of the laser diode 1 intersect at an angle. Therefore, the photodiode 3 can monitor the intensity of light emitted from the back side of the laser diode 1 without returning light reflected from the surface of the photodiode 3 in the direction of the optical axis of the incident light.
In this case, the laser diode 1 is electrically joined to the signal pin 10 piercing the enclosure 9 from the outside through the high-frequency electric line 11 on the ceramic substrate 5, the gold wires 12-15, and the pads 16 and 17 formed on the photodiode carrier 4 and the submount 2 respectively. This enables power supply from the signal pin 10 to the laser diode 1.
In a laser diode module using the above conventional coupling method of a semiconductor laser and an optical fiber, the converging rod lens 20 with refractive index distribution having symmetry about its central axis is used to converge light. Therefore, it has a fault that, even if an angle of incidence of laser light to the core 23 of the optical fiber with a slant end is optimized, high coupling efficiency cannot be achieved. Furthermore, ceramic substrate 5 must be added to supply power to the laser diode 1 with a good frequency characteristic. This results in increase in the number of parts mounted on the base 6, which is unfavorable to assembly.